darwin and modern science-第111章
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； birds and sea currents； as means of transport。 The process will continue； and he concludes：〃At last after a long interval the vegetation on the desolated island will again acquire that wealth of variety and luxuriance which we see in the fullest development which Nature has reached in the primaeval forest in the tropics。〃 （Op。 cit。 page 72。） The possibility of such a result revealed itself to the insight of Darwin with little encouragement or support from contemporary opinion。
One of the most remarkable facts established by Ernst is that this has not been accomplished by the transport of seeds alone。 〃Tree stems and branches played an important part in the colonisation of Krakatau by plants and animals。 Large piles of floating trees； stems； branches and bamboos are met with everywhere on the beach above high…water mark and often carried a considerable distance inland。 Some of the animals on the island； such as the fat Iguana （Varanus salvator） which suns itself in the beds of streams； may have travelled on floating wood； possibly also the ancestors of the numerous ants； but certainly plants。〃 （Op。 cit。 page 56。） Darwin actually had a prevision of this。 Writing to Hooker he says：〃Would it not be a prodigy if an unstocked island did not in the course of ages receive colonists from coasts whence the currents flow； trees are drifted and birds are driven by gales？〃 （〃More Letters〃； I。 page 483。） And ten years earlier：〃I must believe in the。。。whole plant or branch being washed into the sea； with floods and slips and earthquakes； this must continually be happening。〃 （〃Life and Letters〃； II。 pages 56； 57。） If we give to 〃continually〃 a cosmic measure； can the fact be doubted？ All this； in the light of our present knowledge； is too obvious to us to admit of discussion。 But it seems to me nothing less than pathetic to see how in the teeth of the obsession as to continental extension； Darwin fought single…handed for what we now know to be the truth。
Guppy's heart failed him when he had to deal with the isolated case of Agathis which alone seemed inexplicable by known means of transport。 But when we remember that it is a relic of the pre…Angiospermous flora； and is of Araucarian ancestry； it cannot be said that the impossibility； in so prolonged a history； of the bodily transference of cone…bearing branches or even of trees； compels us as a last resort to fall back on continental extension to account for its existing distribution。
When Darwin was in the Galapagos Archipelago； he tells us that he fancied himself 〃brought near to the very act of creation。〃 He saw how new species might arise from a common stock。 Krakatau shows us an earlier stage and how by simple agencies； continually at work； that stock might be supplied。 It also shows us how the mixed and casual elements of a new colony enter into competition for the ground and become mutually adjusted。 The study of Plant Distribution from a Darwinian standpoint has opened up a new field of research in Ecology。 The means of transport supply the materials for a flora； but their ultimate fate depends on their equipment for the 〃struggle for existence。〃 The whole subject can no longer be regarded as a mere statistical inquiry which has seemed doubtless to many of somewhat arid interest。 The fate of every element of the earth's vegetation has sooner or later depended on its ability to travel and to hold its own under new conditions。 And the means by which it has secured success is an each case a biological problem which demands and will reward the most attentive study。 This is the lesson which Darwin has bequeathed to us。 It is summed up in the concluding paragraph of the 〃Origin〃 （〃Origin of Species〃 （6th edition）； page 429。）：〃It is interesting to contemplate a tangled bank； clothed with many plants of many kinds； with birds singing on the bushes； with various insects flitting about； and with worms crawling through the damp earth； and to reflect that these elaborately constructed forms； so different from each other； and dependent upon each other in so complex a manner； have all been produced by laws acting around us。〃
XVII。 GEOGRAPHICAL DISTRIBUTION OF ANIMALS。
By HANS GADOW； M。A。； Ph。D。； F。R。S。 Strickland Curator and Lecturer on Zoology in the University of Cambridge。
The first general ideas about geographical distribution may be found in some of the brilliant speculations contained in Buffon's 〃Histoire Naturelle〃。 The first special treatise on the subject was however written in 1777 by E。A。W。 Zimmermann； Professor of Natural Science at Brunswick； whose large volume； 〃Specimen Zoologiae Geographicae Quadrupedum〃。。。； deals in a statistical way with the mammals； important features of the large accompanying map of the world are the ranges of mountains and the names of hundreds of genera indicating their geographical range。 In a second work he laid special stress on domesticated animals with reference to the spreading of the various races of Mankind。
In the following year appeared the 〃Philosophia Entomologica〃 by J。C。 Fabricius； who was the first to divide the world into eight regions。 In 1803 G。R。 Treviranus （〃Biologie oder Philosophie der lebenden Natur〃； Vol。 II。 Gottingen； 1803。） devoted a long chapter of his great work on 〃Biologie〃 to a philosophical and coherent treatment of the distribution of the whole animal kingdom。 Remarkable progress was made in 1810 by F。 Tiedemann （〃Anatomie und Naturgeschichte der Vogel〃。 Heidelberg； 1810。） of Heidelberg。 Few； if any； of the many subsequent Ornithologists seem to have appreciated； or known of； the ingenious way in which Tiedemann marshalled his statistics in order to arrive at general conclusions。 There are； for instance； long lists of birds arranged in accordance with their occurrence in one or more continents： by correlating the distribution of the birds with their food he concludes 〃that the countries of the East Indian flora have no vegetable feeders in common with America；〃 and 〃that it is probably due to the great peculiarity of the African flora that Africa has few phytophagous kinds in common with other countries； whilst zoophagous birds have a far more independent； often cosmopolitan； distribution。〃 There are also remarkable chapters on the influence of environment； distribution； and migration； upon the structure of the Birds！ In short； this anatomist dealt with some of the fundamental causes of distribution。
Whilst Tiedemann restricted himself to Birds； A。 Desmoulins in 1822 wrote a short but most suggestive paper on the Vertebrata； omitting the birds； he combated the view recently proposed by the entomologist Latreille that temperature was the main factor in distribution。 Some of his ten main conclusions show a peculiar mixture of evolutionary ideas coupled with the conception of the stability of species： whilst each species must have started from but one creative centre； there may be several 〃analogous centres of creation〃 so far as genera and families are concerned。 Countries with different faunas； but lying within the same climatic zones； are proof of the effective and permanent existence of barriers preventing an exchange between the original creative centres。
The first book dealing with the 〃geography and classification〃 of the whole animal kingdom was written by W。 Swainson （〃A Treatise on the Geography and Classification of Animals〃； Lardner's 〃Cabinet Cyclopaedia〃 London； 1835。） in 1835。 He saw in the five races of Man the clue to the mapping of the world into as many 〃true zoological divisions；〃 and he reconciled the five continents with his mystical quinary circles。
Lyell's 〃Principles of Geology〃 should have marked a new epoch； since in his 〃Elements〃 he treats of the past history of the globe and the distribution of animals in time； and in his 〃Principles〃 of their distribution in space in connection with the actual changes undergone by the surface of the world。 But as the sub…title of his great work 〃Modern changes of the Earth and its inhabitants〃 indicates； he restricted himself to comparatively minor changes； and； emphatically believing in the permanency of the great oceans； his numerous and careful interpretations of the effect of the geological chan
